Mapping the parent austenite orientation reconstructed from the orientation of martensite by EBSD and its application to ausformed martensite

A new method is developed for reconstruction of the local orientation of the parent austenite based on the orientation of lath martensite measured by electron backscattered diffraction. The local orientation of austenite was obtained by least squares fitting as the difference between the experimental data and the predicted martensite orientation was minimal, assuming the specific orientation relationship (OR) between martensite and the parent austenite. First, the average OR between austenite and lath martensite was precisely determined and it was shown that both close packed planes and directions between martensite and the parent austenite deviated by more than 1° in low carbon martensite. The quality of the reconstructed austenite orientation map depended strongly on the OR used for the calculation. When Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) ORs were used the austenite orientation was frequently mis-indexed as a twin orientation with respect to the true orientation because of the mirror symmetry of (0 1 1)_α stacking in the K-S or N-W ORs. In contrast, the frequency of mis-indexing was significantly reduced by using the measured OR, where the close packed planes and directions were not parallel. The deformation structure in austenite was successfully reconstructed by applying the proposed method to ausformed martensite in low carbon steel.